The present invention relates to a method and apparatus for testing the ground contact of parts of a networked system.
By way of example, it is known in the field of motor vehicles for a plurality of controllers to be networked in order to allow data to be interchanged between the individual controllers. Owing to the fact that a data protocol is produced, which is sent via one or more lines between the controllers and in which the corresponding data can be entered and read at the appropriate points, it is possible to save a considerable amount of complexity and material in the wiring. Without a data bus a separate cable would be required for each item of information to be interchanged between the respective controllers.
Data are written to and read from the protocol of this data bus by applying a specific voltage level to the line or lines for a specific time period. To do this, it is necessary for these controllers to have a virtually identical reference-ground potential. In the case of a motor vehicle, this is the vehicle ground, with which all the electrical equipment in a motor vehicle makes contact, as an equipotential surface. Networked systems of the type described have become known, for example, as CAN systems.
European patent document EP 0 614 298 A2 discloses a circuit arrangement which is intended to prevent total failure of the entire network if a breakdown in the ground connection or a short to the supply potential occurs on one of the parts in the networked system.
Furthermore, European patent document EP 0 474 907 A1 discloses a method for error identification in a networked system, in which the parts of the networked system are connected to one another in the form of a ring. A wire break at one point in the networked system can be identified in the case of this method by the data originating from one subscriber being sent in both directions. If a receiver receives the data from only one direction, then it is possible to deduce that a wire break is present between the two parts on the path which corresponds to the other direction.
If the ground contact of individual controllers now deteriorates relative to other controllers, disturbances in data transmission can occur since the voltage levels of the individual parts of the networked system are shifted with respect to one another. If the shift is so severe that it is greater than the system tolerance, then there is no longer any certainty that changes in the voltage levels on the data bus can be identified.
One object of the present invention is therefore to identify as early as possible, a deterioration in the ground contact of individual subscribers on a networked system in which data are sent and received via at least one line, with at least one line at each part of the networked system being connected via a resistance arrangement as well as a contact to a common potential of the parts and being connectable via a controllable switch to a further potential.
According to the invention in order to test the ground contact of parts of the networked system, the voltage across the resistance arrangement is compared with a predetermined potential when the line is at one potential in a steady state, and the state of the ground contact of the part is deduced from a comparison of the voltage across the resistance arrangement with the predetermined potential. In this manner, a faulty ground contact can be identified at an early stage, simply and without adversely affecting the ongoing transmission of data. In the case of a CAN network, it is possible to confirm a test of the ground contact by means of the CAN_H line.
According to the invention a method is carried out in which the voltage across the resistance arrangement is compared with a predetermined potential when the line is at one potential in a steady state, the state of the ground contact of the part being deduced from a comparison of the voltage across the resistance arrangement with the predetermined potential.
A faulty ground contact can also advantageously be identified at an early stage in this way using simple means, without adversely affecting the ongoing transmission of data. In the case of CAN network, it is possible to confirm a test of the ground contact by means of the CAN_L line.
In the case of both subjects, a local shift in the voltage owing to a faulty ground contact is compared with the potential on the respective data line. An xe2x80x9caveragexe2x80x9d potential is present on all parts via the connection on the data linesxe2x80x94via the connection at each controller, that is to say each part of the networked system. Any ground fault in an individual subscriber thus has only a minor influence on the potential on the data line. As a consequence of this, in the event of an ground fault in an individual subscriber, a voltage difference is produced across the resistance arrangement, and this voltage difference can be evaluated.
Whenever the controllable switches change over, the potential on the data lines changes at the same time. When such a potential change takes place, overshoots occur which can corrupt the result of any evaluation of the voltage across the resistance arrangement. For this reason, no comparison should take place until the potential on the data line is in a steady state.
Accordingly, in one embodiment of the method according the invention, the potential in the steady state is the common potential. Such a comparison allows ground faults to be detected directly. The comparison when the ground potential on the CAN_H line is in the steady state indicates, advantageously as against a comparison when a VCC potential on the CAN_L line is in the steady state, that more or less undefined voltage losses resulting from the quality of components which are still being used are irrelevant.
In another embodiment of the method according to the invention, the potential in the steady state corresponds to the part-specific potential. In this case as well, undefined voltage losses resulting from the quality of components which are still being used are irrelevant.
In still another embodiment of the method according to the invention, the comparison is carried out with a certain time lag, after which the line is connected, by means of the controllable switches, to a specific potential. The time lag in this case must on the one hand be of such a duration that the steady state is reached, and on the other hand must be of such a duration that the potential has not already switched back again.
In still another embodiment the comparison is carried out within a time window after the switching state of the controllable switches has changed. In this case the time delay which the components experience until the switching process actually takes place is used advantageously. When the state of the transceiver changes, a time period on the order of magnitude of 500 ns passes before the switching process has taken place. The comparison is carried out within this time window, which advantageously ensures that the potential is in a steady state and that the correct potential is present. Thus, the comparison is carried out within the time window while the transceiver is in the xe2x80x9crecessivexe2x80x9d state.
According to another feature of the invention, the battery voltage which can be detected by the individual subscriber is compared with the battery voltage which can be detected by one or more reference subscribers, and a faulty ground contact is deduced if the discrepancy in the detected battery voltages is above a specific threshold value. This method is suitable for detecting a possibly faulty ground contact independently of the check already described. The locally detected battery voltage is sent, for example as information via the bus, to a further controller which, as a reference, itself determines the battery voltage which can be detected there. If the transmitted value of the battery voltage is lower or higher, a ground fault can be deduced.
It should be noted that a reduction in the locally detected battery voltage can also be caused by a faulty contact to the +xe2x88x92connection. For this reason, a further feature of the invention is particularly suitable in combination with the already described methods for verification of the results obtained. In this case as well, the locally detected battery voltage is sent, for example as information via the bus, to a further controller which, as a reference, itself determines the battery voltage which can be detected there. If the transmitted value of the battery voltage is lower or higher, a ground fault can be deduced.
According to another feature of the invention, the contacts of the reference subscriber are designed to be duplicated in order to improve the contact. In this way, a fault source in which a faulty ground contact of the part intended to be used as a reference leads to incorrect results, can be minimized.
In another embodiment of the invention, the comparison of the battery voltage in a motor vehicle is carried out only above a specific rotation speed value. This advantageously avoids incorrect assessments being made as a result of the vehicle""s power supply voltage (and thus the measurable battery voltage) being too low owing to the low rotation speed.
In still another embodiment of the invention, the comparison is activated when a specific minimum load current is flowing in the appropriate part, so that any faulty ground contact also produces a corresponding potential shift.
In yet another embodiment, an entry is made in a diagnostic memory when a discrepancy is identified which exceeds the threshold value relating to the respective subscriber. This allows any faults that have been identified to be read out, for example during routine servicing. The identified faults can be indicated, for fault rectification.
In yet another embodiment, the single-wire reception thresholds are shifted in an appropriate manner when a discrepancy above the threshold value relating to the respective subscriber is identified. Thus, communication is still possible even if individual subscribers have a faulty ground contact.
According to another feature of the invention, when a discrepancy above the threshold value is identified by the respective subscriber, data are transmitted only when the controller is free of load current, thereby avoiding excessive shifts of the voltage level when sending data, caused by the faulty ground contact. Specifically, if a load current is present, the potential shift caused by the faulty ground contact is particularly pronounced.
According to another feature of the invention, a multiple detectionxe2x80x94interrogation of the ground offsetxe2x80x94is carried out, so that dynamic disturbances or superimposed voltages cannot lead to misinterpretation. Thus, for example, ground shifts caused by dynamic events, resulting, for example, from the stalling current of a positioning motor, can be filtered out.
In a further embodiment according to the invention of an apparatus for testing the ground contact of parts in a networked system, the network is implemented by sending and receiving data via two lines. One line at each part is connected via a resistance arrangement and a contact to a common potential for the parts and is connectable via a controllable switch to a further potential. The other line at each part is connected via a resistance arrangement to a part-specific potential of the parts and is connectable via a controllable switch and a contact to a common potential. The voltage drop across at least one resistance arrangement is supplied to one input of a comparator whose other input is supplied with a comparison potential, and the state of the earth contact of the part is deduced from the output signal of the comparator.
This results in a circuit design which can be illustrated comparatively easily and can also be produced cost-effectively in integrated form.
Finally, according to another feature of the invention, the various potentials which are supplied to the inputs of the comparators are generated from a voltage and are supplied to the respective inputs of the comparators via a multiplexer. In this manner, the circuit can be produced with a low level of hardware complexity. The voltages to be measured can also be supplied to an evaluation unit via a multiplexer.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.